UNIT III PSYCHROMETRY AND AIR – CONDITIONING 9

Psychrometric properties, Use of psychrometric chart, Psychrometric process – Sensible heat exchange process, Latent heat exchange process, Adiabatic mixing, Evaporative cooling, Property calculations of air-vapour mixtures.

Principles of air-conditioning, Types of air conditioning systems – summer, winter, year round air conditioners, Concept of RSHF, GSHF, ESHF, Simple problems.

CONCEPT OF PSYCHROMETRY AND PSYCHROMETRICSAir comprises of fixed gases principally, nitrogen and oxygen with an admixture of water vapour in varying amounts. In atmospheric air water is always present and its relative weight averages less than 1% of the weight of atmospheric air in temperate climates and less than 3% by weight under the most extreme natural climatic conditions, it is nevertheless one of most important factors in human comfort and has significant effects on many materials. Its effect on human activities is in fact altogether disproportionate to its relative weights. The art of measuring the moisture content of air is termed “psychrometry”. The science which investigates the thermal properties of moist air, considers the measurement and control of the moisture content of air, and studies the effect of atmospheric moisture on material and human comfort may properly be termed “psychrometrics’’.

DEFINITIONSSome of the more important definitions are given below :1. Dry air. The international joint committee on Psychrometric Data has adopted the following exact composition of air expressed in mole fractions (Volumetric) Oxygen 0.2095, Nitrogen 0.7809, Argon 0.0093, Carbon dioxide 0.0003. Traces of rare gases are neglected. Molecular weight of air for all air conditioning calculations will be taken as 28.97. Hence the gas constant, Rair = 0.287 kJ/kg K Dry air is never found in practice. Air always contains some moisture. Hence the common designation “air” usually means moist air. The term ‘dry air’ is used to indicate the water free contents of air having any degree of moisture. 2. Saturated air. Moist air is said to be saturated when its condition is such that it can co-exist in natural equilibrium with an associated condensed moisture phase presenting a flat surface to it. For a given temperature, a given quantity of air can be saturated with a fixed quantity of moisture. At higher temperatures, it requires a larger quantity of moisture to saturate it. At saturation, vapour pressure of moisture in air corresponds to the saturation pressure given in steam tables corresponding to the given temperature of air.3. Dry-bulb temperature (DBT). It is the temperature of air as registered by an ordinary thermometer (tdb).4. Wet-bulb temperature (WBT). It is the temperature registered by a thermometer when the bulb is covered by a wetted wick and is exposed to a current of rapidly moving air (twb).5. Adiabatic saturation temperature. It is the temperature at which the water or ice can saturate air by evaporating adiabatically into it. It is numerically equivalent to the measured wet bulb temperature (as corrected, if necessary for radiation and conduction) (twb).

6. Wet bulb depression. It is the difference between dry-bulb and wet bulb temperatures (tdb – twb).7. Dew point temperature (DPT). It is the temperature to which air must be cooled at constant pressure in order to cause condensation of any of its water vapour. It is equal to steam table saturation temperature corresponding to the actual partial pressure of water vapour in the air (tdp).8. Dew point depression. It is the difference between the dry bulb and dew point temperatures (tdb – tdp).9. Specific humidity (Humidity ratio). It is the ratio of the mass of water vapour per unit mass of dry air in the mixture of vapour and air, it is generally expressed as grams of water per kg of dry air. For a given barometric pressure it is a function of dew point temperature alone.10. Relative humidity (RH), (φ). It is the ratio of the partial pressure of water vapour in the mixture to the saturated partial pressure at the dry bulb temperature, expressed as percentage.11. Sensible heat. It is the heat that changes the temperature of a substance when addedto or abstracted from it.12. Latent heat. It is the heat that does not affect the temperature but changes the stateof substance when added to or abstracted from it.13. Enthalpy. It is the combination energy which represents the sum of internal and flow energy in a steady flow process. It is determined from an arbitrary datum point for the air mixture and is expressed as kJ per kg of dry air (h).

Note. When air is saturated DBT, WBT, DPT are equal.

PSYCHROMETRIC RELATIONSPressureDalton’s law of partial pressure is employed to determine the pressure of a mixture of gases. This law states that the total pressure of a mixture of gases is equal to the sum of partial pressures which the component gases would exert if each existed alone in the mixture volume at the mixture temperature. Precise measurements made during the last few years indicate that this law as well as Boyle’s and Charle’s laws are only approximately correct. Modern tables of atmospheric air properties are based on the correct versions. For calculating partial pressure of water vapour in the air many equations have been proposed, probably Dr. Carrier’s equation is most widely used.

thermal equilibrium exists with respect to water, air and water vapour, and consequently the air is saturated. The equilibrium temperature is called the adiabatic saturation temperature or the thermodynamic wet bulb temperature. The make-up water is introduced at this temperature to make the water level constant. The ‘adiabatic’ cooling process is shown in Fig. 10.2 for the vapour in the air-vapour mixture. Although the total pressure of the mixture is constant, the partial pressure of the vapour increases, and in the

saturated state corresponds to the adiabatic saturation temperature. The vapour is initially at DBT tdb1 and is cooled adiabatically to DBT tdb2 which is equal to the adiabatic saturation twb2 . The adiabatic saturation temperature and wet bulb temperatures are taken to be equal for all practical purposes. The wet bulb temperature lies between the dry bulb temperature and dew point temperature.

PSYCHROMETRIC CHARTS

The psychrometric charts are prepared to represent graphically all the necessary moist air

properties used for air conditioning calculations. The values are based on actual

measurements verified for thermodynamic consistency. For psychrometric charts the most

convenient co-ordinates are dry bulb temperature of air vapour mixture as the abcissa and

moisture content (kg/kg of dry air) or water vapour pressure as the ordinate. Depending

upon whether the humidity contents is abcissa or ordinate with temperature co-ordinate,

the charts are generally classified as Mollier chart and Carrier chart. Carrier chart having tdb

as the abcissa and W as the ordinate finds a wide application.

The chart is constructed as under :

1. The dry bulb temperature (ºC) of unit mass of dry air for different humidity contents or

humidity ratios are indicated by vertical lines drawn parallel to the ordinate.

2. The mass of water vapour in kg (or grams) per kg of dry air is drawn parallel to the abcissa

for different values of dry bulb temperature. It is the major vertical scale of the chart.

3. Pressure of water vapour in mm of mercury is shown in the scale at left and is the

absolute pressure of steam.

4. Dew point temperatures are temperatures corresponding to the boiling points of water at

low pressures of water vapour and are shown in the scale on the upper curved line. The dew

points for different low pressures are read on diagonal co-ordinates.

5. Constant relative humidity lines in per cent are indicated by marking off vertical distances

between the saturation line or the upper curved line and the base of the chart. The relative

humidity curve depicts quantity (kg) of moisture actually present in the air as a percentage

of the total amount possible at various dry bulb temperatures and masses of vapour.

6. Enthalpy or total heat at saturation temperature in kJ/kg of dry air is shown by a diagonal

system of co-ordinates. The scale on the diagonal line is separate from the body of the chart

and is indicated above the saturation line.

7. Wet bulb temperatures are shown on the diagonal co-ordinates coinciding with heat

coordinates. The scale of wet bulb temperatures is shown on the saturation curve. The

diagonals run downwards to the right at an angle of 30º to the horizontal.

8. The volume of air vapour mixture per kg of dry air (specific volume) is also indicated by a set of diagonal co-ordinates but at an angle of 60º with the horizontal. The other properties of air vapour mixtures can be determined by using formulae (already discussed).In relation to the psychrometric chart, these terms can quickly indicate many things aboutthe condition of air, for example :1. If dry bulb and wet bulb temperatures are known, the relative humidity can be read fromthe chart.2. If the dry bulb and relative humidity are known, the wet bulb temperature can be determined.

3. If wet bulb temperature and relative humidity are known, the dry bulb temperature canbe found.

4. If wet bulb and dry bulb temperatures are known, the dew point can be found.5. If wet bulb and relative humidity are known, dew point can be read from the chart.6. If dry-bulb and relative humidity are known, dew point can be found.7. The quantity (kg) of moisture in air can be determined from any of the following combinations :(i) Dry bulb temperature and relative humidity ;(ii) Dry bulb temperature and dew point ;(iii) Wet bulb temperature and relative humidity ;(iv) Wet bulb temperature and dew point temperature ;(v) Dry bulb temperature and wet bulb temperature ; and(vi) Dew point temperature alone.Figs. 10.4 and 10.5 show the skeleton psychrometric chart and lines on carrier chart respectively.

PSYCHROMETRIC PROCESSESIn order to condition air to the conditions of human comfort or of the optimum control of anindustrial process required, certain processes are to be carried out on the outside air available. The processes affecting the psychrometric properties of air are called psychrometric processes.

These processes involve mixing of air streams, heating, cooling, humidifying, dehumidifying, adiabatic saturation and mostly the combinations of these.The important psychrometric processes are enumerated and explained in the following text 1. Mixing of air streams 2. Sensible heating3. Sensible cooling 4. Cooling and dehumidification5. Cooling and humidification 6. Heating and dehumidification7. Heating and humidification.

Mixing of Air StreamsRefer Figs. 10.6 and 10.7. Mixing of several air streams is the process which is very frequently used in air conditioning. This mixing normally takes place without the addition or rejection of

Process 1-2 : It denotes the cases in which the temperature of the heated spray wateris less than the air DBT.Process 1-3 : It denotes the cases in which the temperature is equal to the air DBT.Process 1-4 : It denotes the cases in which a spray temperature is greater than air DBT. As in the case of adiabatic saturation, the degree to which the process approaches saturation can be expressed in terms of the by-pass factor or a saturating efficiency.

If the water rate relative to the air quantity is smaller, the water temperature will drop significantly during the process. The resultant process will be a curved line such as the dashed 1-4 where 4 represents the leaving water temperature.Note. It is possible to accomplish heating and humidification by evaporation from an open pan of heated water, or by direct injection of heated water or steam. The latter is more common. The process line for it is of little value because the process is essentially an instantaneous mixing of steam and the air. The final state point of the air can be found, however by making a humidity and enthalpy balance for the process. The solution of such aproblem usually involves cut-and-try procedure.

AIR CONDITIONING SYSTEMS

Air conditioning systems require basic arrangement for getting refrigeration effect through

ooling coil followed by subsequent humidification/dehumidification and heating etc. in

order to provide air conditioned space with air at desired temperature and humidity. Air

conditioning systems require different arrangements depending upon the atmospheric air

condition and comfort condition requirement. Such as summer air conditioning systems and

inter air conditioning systems are different. These systems have different arrangement if

outdoor conditions are hot and humid, hot and dry etc. Summer air conditioning system for

hot and dry outdoor condition is given in Fig. 18.20. Here the comfort conditions may

require delivery of air to air-conditioned space at about 25ºC DBT and 60% relative humidity

where the outdoor conditions may be up to 40–44º C DBT and 20% relative humidity in

Indian conditions. Generic arrangement has air blower which blows air across the air filter

between (1) and (2). Air coming out from filter passes over cooling coils and is subsequently

sent for humidification between states (3) and (4). Large size water particles carried by air

are retained by water eliminator. Air finally coming out at state (5) is sent to air conditioned

space. Here psychrometric representation is made considering negligible change in humidity

in water eliminator.